Since the pioneering invention by Vernon Krieble in the 1950s, anaerobic adhesives have been used as thread lockers. Early on, the unique property of these adhesives to cure on exclusion of air was recognized as especially suited for automated application of an adhesive to a nut, bolt or pipe thread long prior to assembly. In order to practically manufacture and handle such articles, however, the early liquid adhesive formulation had to be modified so that it could stay in place after application in an essentially tack-free condition. A variety of formulations have been invented which achieve this objective. The various approaches include dissolving or dispersing the liquid anaerobic formulation into a thermoplastic matrix; coating the liquid adhesive with a solid polymer shell; or entraining the anaerobic adhesive in a solid film laid down from an emulsion or a solvent solution.
Competition for these pre-applied thread locking formulations comes from other adhesive systems which mimic the cure-on-assembly feature of anaerobic adhesives. The competitive adhesives are essentially two part materials, kept separate from each other on the part but mixed on assembly. Most typically, this involves encapsulating a component of the cure system of the adhesive in microcapsules which are then dispersed throughout the body of the adhesive. Upon assembly of the parts the microcapsule is broken and the cure component blended with the remaining ingredients of the adhesive. Epoxy adhesives, and acrylic 2adhesives using cure systems which are too active to be oxygen stabilized, are the most common of these types of adhesives. Other common two part adhesive systems may also be used such as urethane adhesive systems. condensation cured silicones, or hydrosilation cured silicones.
For the purposes of this application these various adhesive or sealant formulations, which can be pre-applied to a substrate to produce a substantially dry-to-the-touch formulation adapted to cure upon assembly of the substrate with a mating substrate will be referred to generally as "pre-applied" adhesives or sealants. References disclosing pre-applied adhesive or sealant formulations include U.S. Pat. Nos. 3,489,599, 3,547,851, 3,625,875, 3,757,828, 3,814,156, 3,826,673, 3,922,449, 4,417,028, 4,497,916, and 4,632,944 all assigned to Loctite Corporation or Loctite (Ireland) Ltd.; and U.S. Pat. Nos. 4,325,985, and 4,428,982, both assigned to Oakland Corporation.
Although pre-applied adhesives have found their predominant use on threaded surfaces such as fasteners or pipes, they may also be used on articles such as rivets or flat mated surfaces.
Liquid anaerobic sealants have also been used to provide gas and liquid tight seals between interference fitted parts. In the mid 70s automobile engine manufacturers began using liquid anaerobic sealants to seal engine cup-plugs (sometimes also called engine core plugs or ice plugs) used to seal holes in the side of engine castings. The holes are placed in the engine casting so that the core sand can be removed from the castings during manufacturing. The plugs are usually made from stamped sheet metal in the shape of a shallow cup. They are press-fitted into the engine holes with a typical designed interference fit of 0.006-0.012 inches. The liquid anaerobic sealant is typically applied to the holes in the engine block just prior to assembly. The liquid adhesives were found not only to effectively seal leaks but also to substantially increase the average blow-out strength of the plugs.
For a number of years now automotive manufacturers have been asking for a pre-applied product which can be used instead of the liquid products currently used for cup plug sealing. Numerous attempts have been made by various manufacturers that develop such a product. To date all have failed when it came to interference fits because the pre-applied resin on the periphery of the cup plug is shaved off at the interference point.